Non-geosynchronous orbit (NGSO) satellites are satellites that continuously move with respect to the earth. To communicate with these satellites, fixed ground terminal receive antennas must continuously track a satellite until it goes out of its tracking range. A hand-off is the process of tracking one given satellite and then the next satellite in the constellation so that communications continue. The most desirable is a soft hand-off in which communications with the next satellite is made prior to breaking communications with the current satellite. Therefore, to perform a soft hand-off, the ground terminal receive antenna must track and communicate with the current satellite while it is simultaneously establishing a communications link with the next satellite.
An active electronically scanned array (AESA) with dual independently steered receive beams is capable of performing a soft hand-off. However, the cost for the size of the array required to provide adequate gain at maximum scan for many non-geosynchronous orbit satellite systems is prohibitive.
A single parabolic dish antenna with a two-dimensional mechanical positioner cannot perform a soft hand-off since it can only communicate with one satellite at a time. The loss of signal while the dish mechanically scans to the next satellite and establishes link is generally unacceptable. The process of scanning to the next satellite, called flyback, and performing a soft hand-off may be achieved by a dual dish system where one dish tracks and communicates with the current satellite and the other dish locates, tracks and establishes link with the next satellite. Although dual dish systems are capable of performing a soft hand-off, there is the added cost and size penalty of the second dish and the two-dimensional mechanical positioner.